1. Field of the Invention
The present invention relates to the field of integrated circuits and more particularly to a high voltage switch and capacitively coupled drive circuit for providing a high voltage relay with a controlled input which is suitable for fabrication on a single silicon chip. The present invention also relates to the field of solid state telephone line circuits in which integrated circuit DC relays and capacitively coupled drive circuitry is required to couple DC input voltages to high voltage switches while maintaining isolation between the high voltage switch and the controlled DC input to the switch.
2. Description of the Prior Art
Prior art high voltage switches for deriving a high voltage output signal from a DC input in which the switch portion of the circuit is required to be fabricated on an integrated circuit silicon chip MOS configuration, for example, must maintain electrical isolation between the solid state relay contacts of the high voltage switch and the control input by which a DC or pulsed DC voltage is coupled to the circuit. In the prior art, this electrical isolation has been achieved by means of optical couplers or transformers. Neither optical couplers nor transformer circuitry can be combined with the high voltage switch on a single integrated circuit chip using current known integrated circuit fabrication techniques for MOS integrated circuit design. Isolation between the solid state relay output of the high voltage switch and its control input is essential in an integrated circuit configuration since the high voltage at the relay output would destroy the input logic portion of the circuit, causing undesired switching of the relay. Additionally, the isolation of the output terminals of the high voltage switch and its control input is particularly suitable in applications such as telephone line circuit configurations in which a floating ground at the switch is required.
As aforementioned, optical couplers must be manufactured separately and put together as an assembly rather than being fabricated on a single silicon chip for the transmission of signals between two circuits which must be electrically isolated from each other. Such optical couplers typically include a light emitting diode and photosensitive transistors configured so that an input voltage is applied to the LED terminals. The resulting LED light output then turns on the photosensitive transistors thereby producing an electrically isolated output voltage at the phototransistor output from the input voltage. Such optical couplers are well known and are further described in Modern Microelectronics by Research and Education Association, Second Printing 1974, at pages 312 and 313 thereof.
The present invention may be advantageously utilized in MOS circuitry fabricated in accordance with well known dielectric isolation processes such as are described in greater detail, for example, in the aforementioned Modern Microelectronics, Second Printing 1974, at pages 425 thru 434. A particular known fabrication technique in which V shaped curves are etched into the surface of the metal oxide semiconductor dice (VMOS) is a well known fabrication technique in which the vertical difference between two diffusion layers (the channel length) is maintained short, ie., a few microns, is one exemplary MOS logic technology which is suitable for fabricating the circuit of the present invention and in which technology the prior art optical couplers can not be integrated. Since these fabrication techniques are well known, they will not be described in detail herein.
The disclosed circuit without prior art optical couplers or transformers can also be fabricated in large scale integrated circuits (LSI) together with other devices and is particularly suitable and cost effective for use in such LSI circuits which are designed to be manufactured in large quantity, such as the signal generator portion of telephone line circuits of the type described by the copending U.S. patent application Ser. No. 773,713 of Robert Treiber, filed March 2, 1977 and assigned to the same assignee as the present application.